1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system including the diffractive optical element. For example, the present invention is suitable for optical instruments such as a video camera, a digital camera, a TV camera, a telescope, and binoculars.
2. Description of the Related Art
As opposed to a method of reducing chromatic aberration of a lens system (optical system), by way of combination of glass materials, there has been conventionally known a method of reducing chromatic aberration of a lens system (optical system) by way of the use of a diffractive optical element in which a part of a surface of a lens or a flat plate is provided with a diffraction grating portion (diffractive optical portion) having a diffraction effect.
This method of reducing the chromatic aberration with the use of the diffractive optical element utilizes such a physical phenomenon that the chromatic aberration with respect to a beam having a given reference wavelength occurs in opposite directions between at the refracting surface and at the diffracting surface in an optical system. Further, the diffraction grating portion can have an effect similar to that of an aspherical lens by appropriately changing the period of periodic structure of its diffraction grating. Accordingly, the diffraction grating portion is also effective in reducing various aberrations other than the chromatic aberration. In general, the diffraction grating has blazed structure including grating surfaces and grating side surfaces. The diffraction grating having the blazed structure is capable of efficient beam diffraction with respect to diffracted beams in one particular order (hereinbelow, referred to as “particular order” or “design order”) and a particular wavelength.
There is known a diffraction grating portion structured so that the diffraction efficiency in the particular order can be obtained at a sufficiently high level across the entire visible wavelength band. Specifically, two diffraction gratings are arranged in close contact with each other, and, as materials to form the respective diffraction gratings, a material of low refractive index and high dispersion and a material of high refractive index and low dispersion are used. Then, by appropriately setting the height of the diffraction grating, high diffraction efficiency is achieved in a wide wavelength band with respect to diffracted beams in a desired order. Hereinbelow, such a diffraction grating portion is referred to as “contacting two-layer DOE”. Here, the “DOE” is an abbreviation for “diffractive optical element”. Further, by arranging multiple diffraction gratings in a stacked manner, and also appropriately setting the materials of the respective diffraction gratings and the heights of the respective diffraction gratings, high diffraction efficiency is achieved in a wide wavelength band with respect to the diffracted beams in the desired order. Hereinbelow, such a diffraction grating portion is referred to as “stacked DOE”. Note that, the diffraction efficiency is expressed by a ratio of the light quantity of diffracted beams in each order to the light quantity of entire transmitted light fluxs.
In the diffractive optical elements disclosed in Japanese Patent Application Laid-Open No. 2004-78166 and Japanese Patent Application Laid-Open No. 2008-241734, in order to obtain a diffraction efficiency equal to or higher than 99% across the entire visible wavelength range, there is used a material whose partial dispersion ratio θgF has a smaller value (linear dispersion characteristic) compared to the normal material.
Of the materials which enable high diffraction efficiency and are thus suitable for forming the diffraction grating, a material in which indium-tin oxide (ITO) nanoparticles are dispersed in a resin is known as a material having the linear dispersion characteristic.
Unlike other inorganic oxides, the refractive index of ITO changes due to free carriers generated by the doping of tin and the holes of oxygen in addition to due to electron transition. Due to the electron transition and the free carriers, ITO has an extremely strong linear dispersion characteristic. By the way, ITO is known as a material having relatively high transmittance and is thus used for transparent electrodes, for example. However, the transmittance of ITO is not sufficient when ITO is used for an optical system which requires transmittance higher than such level. The decrease in transmittance of ITO is caused by the doping of tin. For this reason, a material which is extremely transparent and also has the linear dispersion characteristic is very difficult to obtain.
If a larger amount of the ITO nanoparticles is used as a material for the diffraction grating in an attempt to obtain high diffraction efficiency, the transmittance of the diffraction grating portion becomes lower, and thus application of the diffractive optical element to an optical system becomes unpractical. Conversely, if the mixing ratio of the ITO nanoparticles contained in the material of the diffraction grating is decreased in order to increase the transmittance for the purpose of applying the diffractive optical element to an optical system, high diffraction efficiency becomes difficult to obtain. In addition, the grating portion having a small grating height is difficult to realize. Hence, when the diffractive optical element is applied to an optical system, it is important that the percentage content of the ITO nanoparticles contained in the material of the diffraction grating be made as small as possible, and also that the grating height (grating height) of the grating portion forming the diffraction grating be made small (low).